This invention relates to a multivoice demodulating circuit for a TV multivoice receiver for receiving a TV multivoice broadcast, the receiver being intended for mobile use.
FIG. 3 shows a conventional circuit of the sort described above.
The conventional circuit shown in FIG. 3 comprises an input terminal 1 for receiving a composite signal obtained through FM detection by an audio detector (not shown). A low-pass filter (LPF) 2 allows a main signal (L+R) or main audio signal in the composite signal to pass therethrough. A band-pass filter (BPF) 3 allows carriers in the composite signal at 2 f.sub.H (f.sub.H =15.75 kHz) and at 3.5 f.sub.H to pass therethrough. A sub-signal demodulating circuit 4 receives the 2 f.sub.H carrier drawn from the band-pass filter 3 and provides a subsignal (L-R) or subaudio signal as its output. A Q signal indentifying circuit 5 receives the 3.5 f.sub.H carrier from the band-pass filter 3. The Q signal is a signal superimposed on the composite signal which identifies the mode of the broadcast, e.g., monaural, stereo or multi-lingual. A mode setting circuit 6 generates a mode switching signal derived from the signal generated by the Q signal identifying circuit 5 and a mode setting signal. A matrix circuit 7 controlled by the mode switching signal received from the mode setting circuit 6 is used to process the main signal or main audio signal L+R drawn from the lowpass filter 2 and the sub-signal or subaudio signal L-R from the sub-signal demodulating circuit 4 so as to produce the audio signals L and R distributed to left and right channels through de-emphasis circuits 8 and 8'. The form of the outputs L and R depends upon which mode has been set. In the case of multi broadcasts, the user specifies by the mode setting signal which voice signal is to be heard.
In the circuit thus arranged, the mode setting circuit 6 draws a 922.5 Hz (multi) or 982.5 Hz (stereo) control signal by AM-detecting the 3.5 f.sub.H carrier drawn from band-pass filter 3 and controls the matrix circuit 7 using the mode setting signal and the identifying signal from the Q signal identifying circuit 5 for distinguishing between the stereo and multi modes based on the control signal.
However, the sub-audio signal or the sub-signal is strongly affected by various types of noise to an extent greater than that to which the main signal or main audio signal is affected. The sub-signal only may be generated particularly in the multi mode and is thus disadvantageous against noise compared with the case of the stereo mode wherein the main signal and the sub-signal are generated in the form of a composite signal.
Attempts may be made to reduce the influence of noise by limiting the band. In that case, almost no problem is posed while conversation is broadcast but the tone sensitivity is extremely impaired when music is being broadcast.
When an auto separation control is simultaneously employed, moreover, the sub-audio signal or the sub-signal is strongly affected by noise because the attenuation of the sub-signal must be suppressed because of a feeling of physical disorientation.